The present invention relates to a speaker system, in particular to a speaker system for use in an automobile vehicle.
It has been known that a speaker system for use in an automobile vehicle is usually a multi-speaker system, such as that shown in FIG. 5.
Referring to FIG. 5, a conventional multi-speaker system comprises a plurality of speakers 4 disposed in a front section, on side doors 2 and on a rear tray 3 within an automobile vehicle 1. With the use of such an arrangement, it is possible for an entire area within the vehicle to obtain a uniform sound pressure, enabling all the persons staying in the vehicle to easily and similarly enjoy music or the like.
However, there is a problem with the conventional multispeaker system illustrated in FIG. 5. Namely, in use of the speaker system, there are primary sounds emitted directly from individual speakers 4 and secondary sounds generated when the primary sounds are reflected from inner walls of the automobile vehicle. Since the primary sounds and the secondary sounds will be unavoidably synthesized together, there is always a fluctuation in the frequency characteristic of the multi-speaker system (due to a reflection synthesizing effect), resulting in a complex peak/dip phenomenon in audible frequency bands.
To alleviate the problem caused by the reflection synthesizing effect, it is necessary to simplify the sound sources involved in a multi-speaker system. Namely, the number of speakers (serving as sound sources) should be made as fewer as possible and these speakers should be arranged in only one direction. Further, to ensure an audio effect similar to a music listening room, it is important to satisfy the following requirements.
(a) A sound image should be located in the front.
(b) A sound image deviation possibly caused due to different seat positions within an automobile vehicle should be eliminated.
(c) A peak/dip phenomenon possibly caused due to the reflection synthesizing effect should be eliminated.
In the multi-speaker system shown in FIG. 5, since the speakers are disposed in every direction, it is difficult to have the sound image located in the front within a vehicle. Therefore, it is understood that the sound image may be located in the front only when the speakers are located in a front section in a vehicle. In other words, if an automobile speaker system is used which includes a pair of speakers disposed only on opposite sides of a front section in a vehicle, it is sure to obtain some improvement in locating the sound image in the front within an automobile vehicle.
Nevertheless, there is another problem with an automobile speaker system including a pair of speakers disposed only on opposite sides of a front section in a vehicle.
Namely, in a limited space within an automobile vehicle, as shown in FIG. 3a (which is a graph indicating a relationship between sound pressure and frequency), the sound pressure will reach a peak when the frequency is about 70 Hz. Further, as shown in FIG. 4a, the sound pressure will also reach a peak in positions close to rear seats within a vehicle. Since there is such a peak characteristic, sound signals produced by speakers will be somehow unclear in the vicinity of the rear seats.
In fact, FIG. 4a indicates sound pressure levels detected in all different positions within a vehicle at a moment a sound signal having a low frequency (such as 70 Hz) is emitted from speakers located in the front section of the vehicle. As shown in FIG. 4a, a sound signal having a low frequency is detected to have a uniform sound pressure in a lateral direction across the vehicle, but a sound pressure in the rear section will be higher than that in the front section in the vehicle.
In order to inhibit the occurrence of a peak in sound pressure, there has been suggested an improved speaker system as shown in FIG. 6. Referring to FIG. 6, this speaker system includes a two-channel source 6, a pair of speakers 7,7 connected to the source 6 through two signal lines, a pair of amplifiers 9,9 connected before the two speakers 7,7 on the two signal lines. Further, a pair of filters 8,8 are connected between the two-channel source 6 and amplifiers 9,9 on the two signal lines to attenuate low frequency components.
With the use of the filters 8,8 shown in FIG. 6, it is possible to improve the quality of a sound reaching specific seats (for example, rear seats), but this will undesirably attenuate the level of a low frequency sound signal reaching other seats (for example, front seats). For instance, if a low frequency signal from the above channel source 6 is attenuated to improve the quality of a sound reaching the listeners sitting in the rear seats in the vehicle, the level of a low frequency component of a sound signal reaching the listeners sitting in the front seats will also be unavoidably reduced. This means, the use of the two filters 8,8 will undesirably cause vanishing of a bass feeling for the listeners sitting in the front seats.
As a result, a conventional speaker system shown in FIG. 6 has been proved unable to obtain an improved sound quality throughout the entire internal space of an automobile vehicle.